Pressurized Water Reactor-PWR
The pressurized water reactor is the most popular type of reactor. There are more than 300 PWRs used in nuclear power stations and further 100 built into different vehicles. By this type of reactor, the primary coolant is under high pressure (100-150 atm), that is why it boils at about 200-300°C. The transmission of heat from the water of the primary coolant to the secondary coolant happens inside heat exchangers. This heat is going to be led to the turbines.
The advantage of this method is that the primary coolant, which contacts the reactor core and this way becomes radioactive, flows in a closed system.

The performance of pressurized water reactors is regulated in another way than that of the boiling water reactors. Because the water doesn’t boil, there is no negative feedback. It is controlled with control rods and boric acid mixed in the coolant, due to the boron being a good neutron absorber. With the reduction of its concentration, the neutron absorption decreases as well, while the performance grows and vice versa.  Therefore, with burning out the fuel, the concentration of the boron diminishes. Control rods are only used for changing the performance.

Fast Breeder Reactor-FBR
The FBR is the type of reactor that produces more nuclear fuel than it uses up. In this case the chain reaction is sustained by fast neutrons, without the use of moderators. To start such a reactor uranium is needed, enriched by 15-20%, but once it starts, it produces the fuel for itself.

The fuel is 20% plutonium dioxide and 80% uranium dioxide. The plutonium is fissionable, its fissure can be achieved with fast neutrons. The neutron obtained from the fissure is also fast, so the circle is closed. The reactor core is surrounded with clean uranium-238, which, by neutron capturing and by two beta decays, turns into plutonium, later processed and used as fuel.

In these types of reactors liquid metal is often used as coolant. The most commonly used metal is sodium, but in smaller plants NaK and lead are also in use. Some of early FBRs used mercury. One advantage of mercury and NaK is that they are both liquids at room temperature, which is convenient for experimental rigs but less important for pilot or full scale power stations.

Pool Type Reactor
The pool type reactor is an experimental reactor which has its core immersed in an open pool of water.  The water acts as neutron moderator, cooling agent and radiation shield. The layer of water above the reactor core shields the radiation so completely that operators may work above the reactor in total safety. This design has two major advantages: the reactor is easily accessible and the whole primary cooling system is under normal pressure. This kind of reactor serves mainly for neutron source in experiments but is also used in medicine.
Pressurized Heavy Water Reactor-PHWR or CANDU
The CANDU reactor uses high pressurized heavy water as moderator and as coolant as well. Having heavy water as moderator, it works with natural uranium. This simplifies the preparation of the fuel. CANDU is the abbreviation for CANada Deuterium Uranium.
The CANDU type has many advantages, in contrast with traditional pressurized water reactors:

• The reactor vessel is covered with hundreds of tubes. The fuel rods are placed in these tubes and therefore are easily accessible allowing their replacement during the operation.  
• The fuel rods can be easily moved – depending on how much fissuring nuclei they still contain.
• The reactor doesn’t have to bear any pressure, because only in the tubes is the moderator under high pressure.
• Since the pressure and temperature are so low, the sensors can follow the process much more easily.
• It works with less natural uranium, but the great amount of heavy water needed means serious initial expenditures.

Naval reactors
In the marine, reactors are used to drive bigger vessels, mother ships and submarines. With the steam produced by the reactor, a steam turbine is fed which propels directly the screw or an electric generator (in this case the screw is driven by an electric engine).
The reactors are usually pressurized water or liquid metal cooled reactors, which differ in many ways from those that generate electric current, like in the size of the reactor (it has to fit in a small place) or in the fuel used. In the case of nuclear submarines highly enriched uranium is the fuel (not uranium dioxide, but the alloy of uranium and zinc) and so there is no need for a refill all along its operating time, and its range is arbitrary. Furthermore, when dived under water it can still reach high speeds and great arbitrary, because there is no need for sucking in air to drive the submarine’s diesel engines.
Until the middle 90s, in civic shipping, they were only used in ice-breakers. Three of these ships were built in the Soviet Union and were able to travel from Murmansk to the Bering Sea without refilling their tanks – in extremely difficult conditions as well.